It's in the dev diary.
Link
So what is this Christmas Market, and how does it work? It can be placed anywhere on your island, where it can buff the output of all workyards in an area around it. The bonus it provides, as well as the area to which it covers, can be increased by upgrading & unlocking further buffs for the building. If placed correctly, the potential benefits are extremely lucrative! You also don't need to fret about fully upgrading it this year, as you'll be able to continue doing so in every future Christmas Event.